In many applications it is desirable to generate an electrical signal in response to a force input. It is particularly desirable to be able to generate a signal having a signal strength proportional to the force input. Specific applications abound. For example, turning to FIG. 1, a bicycle 50 with an electric motor 52 is shown. It is desirable to be able to control the speed of the electric motor such as for example by the rider being able to press her thumb against a force sensing button, thus producing a stronger signal which may be used to control the speed of the electric motor. Such a desirable configuration is shown in FIG. 2 wherein the operator's thumb is shown in phantom over speed control button 5. Other applications where it is desirable to produce an electrical signal proportional to a force input include golf carts or any driven vehicle, airplane controls, and cursor positioning and screen scrolling controls in personal computer applications. A further application where it would be useful to produce a output signal responsive to a force input would for example be in wireless control units for home entertainment, as for example to control the volume, tone, and rate at which channels on a television channel selector can be adjusted. That is, the harder the button is pressed, the faster the channels would change. A further use of the circuit would be in automotive applications, such as a horn switch, environmental controls (heating, air conditioning), and entertainment system controls.
Circuits which may perform the desirable function of providing an output signal proportional to a force input are known. However, such circuits have certain limitations.
The first limitation of the prior art circuits is that it is often desirable to have a voltage range available for the output signal which exceeds the capacity of the force sensor. In certain applications, an output voltage range of 5 volts is desirable. However, many force sensors may only withstand a continuous voltage of approximately 1 volt. Therefore, the circuit must be designed for only a 20 percent duty cycle by adding additional components to the circuit (typically a microprocessor), thereby significantly increasing cost of the circuit. It is desirable, however, to keep the cost of the circuit low, for obvious reasons.
An additional problem with the prior art is that it is desirable to produce an output voltage signal, however most force sensors produce a varying current in response to force applied to the sensor. The current must therefore be converted to an output voltage. This is done by an operational amplifier (op amp) configured as a current-to-voltage converter. This provides a negative voltage, which must be inverted to provide a positive voltage for the output. A positive voltage is desirable to be able to perform an analog to digital conversion at the output. Inverting the negative voltage requires the addition of a negative supply voltage generator, as well as relatively expensive op amps.
In addition to being simple and therefore relatively inexpensive and simple to manufacture, a force sensitive circuit should be relatively temperature stable so that it may be used in a range of temperatures to be experienced. For example, in the example where the force sensitive circuit is used to control the speed of an electric motor connected to a bicycle, ambient temperatures may vary between 0.degree. C. and 40.degree. C., and beyond.
Therefore, what is needed in the art is a relatively inexpensive, easy to manufacture circuit which produces a voltage output in response to the force input, avoiding the problems of voltage limitations across the sensor as well as the need to invert negative voltages and compensate for temperature variations.